The ability to detect, quickly and reliably, the presence or absence of specific chemicals can be a matter of life or death. Leaks of toxic gases in air, monitoring of glucose in the bloodstream, testing for harmful compounds in foods and water, and early alert of chemical and biological warfare agents all require reliable and sensitive sensing devices. For example, the ability to detect, quickly and reliably, the presence of trace amounts of explosives or chemical and biological warfare agents in air is an urgent demand of homeland security.
Several detection devices and accompanying methods have been proposed and developed as part of a body of prior art. One example of a prior art sensor device is the quartz crystal microbalance. A quartz crystal microbalance is a disk-like quartz crystal. When an analyte (chemical or biological species) absorbs or binds onto the crystal surface, the effective mass of the crystal increases and results in a decrease in the resonance frequency that can be accurately measured. Using a quartz crystal microbalance, however, is subject to inherent limitations. Quartz crystal microbalance sensors detect changes in mass. By using a method to detect changes in mass, one cannot easily discriminate specific binding due to the analyte from nonspecific bindings due to other molecules. The smallest amount of mass change, a parameter that describes the sensitivity of a mass-detection based sensor, is also limited.
Another widely used mechanical device for chemical or biological sensors is the microfabricated cantilever, or microcantilever. Microcantilever sensors can be operated either in AC and DC modes. In AC mode, the cantilever is set to oscillate and the resonance frequency is detected upon binding of an analyte onto the cantilever surface. The operation of detecting the applicable resonance frequency of microcantilever sensors is similar to the detection operation performed using the quartz crystal microbalance. In the direct current (DC) mode, a bending in the cantilever induced by analyte adsorption is detected. The bending of the cantilever arises from analyte adsorption-induced surface stress in the cantilever.
Microcantilever sensors, like quartz crystal microbalance sensors, have inherent limitations. In both AC and DC modes, it is necessary to be able to detect the mechanical movement of the microcantilever, which is normally achieved in two ways, optical method and piezoresistive detection. The optical method requires the use of laser diodes and photodetectors in addition to the employment of complex electronic circuits. Mechanical adjustments of the laser beam and photodetectors is also required, which are not desirable for many practical devices. Piezoresistive detection uses a cantilever whose resistance is sensitive to mechanical bending. Piezoresistive detection allows for electrical detection of a mechanical response of the cantilever without using external optics. However, piezoresistive cantilevers have limited sensitivity and consume considerable amounts of power during operation. Moreover, scaling microcantilever sensors down to the nanometer scale remains a difficult task.
While many sensor devices and accompanying methods have been proposed and developed, a device that can, for example, cheaply and effectively satisfy the demands of homeland security has yet to be introduced due to a number of practical issues. A need exists for a power-efficient, stable and accurate apparatus and method to detect chemical and biological analytes.